Volatile-rich Sub-Neptunes as Hydrothermal Worlds: The Case of K2-18 b

Temperate exoplanets between the sizes of Earth and Neptune, known as “sub-Neptunes”, have emerged as intriguing targets for astrobiology.

It is unknown whether these planets resemble Earth-like terrestrial worlds with a habitable surface, Neptune-like giant planets with deep atmospheres and no habitable surface, or something exotic in between. Recent JWST transmission spectroscopy observations of the canonical sub-Neptune K2-18 b revealed ~1% CH₄, ~1% CO₂, and a non-detection of CO in the atmosphere.

While previous studies have proposed that the observed atmospheric composition could help constrain the lower atmosphere conditions and determine the interior structure of sub-Neptunes like K2-18 b, the possible interactions between the atmosphere and a hot, supercritical water ocean at its base remain unexplored.

In this work, we investigate whether a global supercritical water ocean, resembling a planetary-scale hydrothermal system, can explain these observations on K2-18 b-like sub-Neptunes through equilibrium aqueous geochemical calculations.

We find that the observed atmospheric CH₄/CO₂ ratio implies a minimum ocean temperature of ~715 K, whereas the corresponding CO/CO₂ ratio allows ocean temperatures up to ~1060 K. These results indicate that a global supercritical water ocean on K2-18 b is plausible.

While life cannot survive in this ocean, this work represents the first step towards understanding how a global supercritical water ocean may influence observable atmospheric characteristics on volatile-rich sub-Neptunes.

Future observations with better constrained NH₃ and CO mixing ratios could further help distinguish between possible interior compositions of K2-18 b.

Cindy N. Luu, Xinting Yu, Christopher R. Glein, Hamish Innes, Artyom Aguichine, Joshua Krissansen-Totton, Julianne I. Moses, Shang-Min Tsai, Xi Zhang, Ngoc Truong, Jonathan J. Fortney

Comments: 15 pages, 5 figures, 1 table
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2409.06258 [astro-ph.EP] (or arXiv:2409.06258v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2409.06258
Focus to learn more
Submission history
From: Xinting Yu
[v1] Tue, 10 Sep 2024 07:00:42 UTC (2,350 KB)
https://arxiv.org/abs/2409.06258
Astrobiology